<html>
<head>
<title>test1</title>
<style>
#scrollto{
	position:fixed;
	width:60px;
	height:60px;
	position:fixed;
    left:50%;
    margin-left:502px;
    bottom:20px;
   // _position:absolute;
    //_top:expression(eval(document.documentElement.scrollTop || document.body.scrollTop) +eval(document.documentElement.clientHeight || document.body.clientHeight) -305+'px');
    z-index:998;
}
</style>
</head>
<body>
	<div id = "scrollto"  >
		<input type="button" value="ok" onclick="window.onscroll()" />
	</div>
	
	
<script>
	window.onscroll = function () {
    if (document.documentElement.scrollTop + document.body.scrollTop > 1000) {
        document.getElementById("scrollto").style.display = "block";
    }
    else {
        document.getElementById("scrollto").style.display = "none";
    }
}
</script>
</body>
</html>
